Processes for the production of glass-clear sheets from thermoplastic plastics by the radical addition polymerization of olefinically unsaturated monomers are known. They are based on the use of monomers which represent a solvent for the polymers thereof. Examples of such monomers are the esters of acrylic acid and methacrylic acid. In particular, processes for the production of glass-clear moldings by the bulk polymerization of methyl methacrylate have acquired commercial significance.
Despite the favorable optical and mechanical properties thereof, these materials are unsatisfactory on account of the brittleness thereof, i.e. the low resistance thereof to impact. Accordingly, there has been no shortage of attempts to produce polymethyl methacrylate moldings having the same favorable optical properties, but considerably improved impact strength and with the level of dimensional stability to heat substantially intact. Of the methods used for the high-impact modification of polymethyl methacrylate or methyl methacrylate copolymers, alloying with polyurethane elastomers is of particular interest because the properties of such combination materials are variable over wide ranges by virtue of the wide range of possible starting materials and variants of the production process.
It is known that cross-linked or linear polyurethane elastomers containing unsaturated or olefinic double bonds on the basis of polyether or polyester polyols and low molecular weight, aliphatic polyhydric alcohols may be produced by polyaddition in solution in vinyl monomers whose polymerization would normally lead to hard polymers having a high glass transition temperature, for example in methyl methacrylate or in monomer mixtures consisting predominantly of methyl methacrylate.
German Patent Publication No. 2,003,365 describes a process for the production of high-impact moldings from thermoplastic polymers, in which a cross-linked polyurethane is synthesized in a vinyl monomer or in a mixture of vinyl monomers by reacting polyisocyanates with polyfunctional compounds containing Zerewitinoff-active hydrogen atoms, after which the vinyl monomer is polymerized. Since, in this process, gels which may neither be cast nor shaped are obtained as an intermediate stage, shaping has to be completed before the polyaddition reaction has advanced to the cross-linked polyurethane stage. Accordingly, such a process may only be combined with difficulty with the now generally accepted process for the production of moldings, particularly sheets, by bulk polymerization.
In the course of further development of the above proposal, it was found that gelation of the polyurethane solutions could be avoided by selecting the polyurethane precursors for the functionality thereof in such a way that the concentration of the branching or cross-linking sites did not exceed a certain limit (German Patent Publication No. 2,312,973). In this process, however, it is not possible to rule out the danger of the formation of gels and other inhomogeneities which adversely affect the quality of the completed polymer products.
German Patent Publication No. 2,033,157 describes a process for the production of high-impact rigid polymers based on acrylic esters in which solutions of linear polyurethanes containing unsaturated groups accessible to copolymerization are produced from diisocyanates, aliphatic diols, relatively high molecular weight aliphatic polyester diols and isocyanate-monofunctional compounds containing an ethylenic double bond in a monomeric acrylic and/or methacrylic ester.
It has been found that the composite materials formed during the bulk polymerization of such solutions are made up of two phases, the polyurethane component representing the continuous phase and the polymethacrylate component the disperse phase, even with relatively low concentrations of polyurethane.
The observed phase structure makes it necessary to use polymers having optimal mechanical properties, particularly with regard to tensile and tear strength, elongation at break and elasticity and in regard to the dependence thereof upon temperature. It is known that these properties and the thermal stability under load of polyurethane elastomers may be improved by incorporating urea groups as so-called "hard segments" into the polymer chain. This may be done by using diamines in the synthesis of the elastomer, aliphatic and cycloaliphatic diamines being particularly suitable on account of the imperative color stability and resistance to weather.
However, methyl methacrylate or even less polar acrylic and methacrylic acid esters are relatively poor solvents for polyurethane urea elastomers of this type, as reflected, for example, in the very steep increase in solution viscosity with increasing content of urea groups in the polyurethane chain. The hard segments of the elastomer chains form crystalline associates which are firmly bound through hydrogen bridges and which cannot be kept in solution by poor solvents. This crystallization of the hard segments frequently results in clouding of the polyurethane monomer solutions and the moldings obtained from them by polymerization. Although the most widely used polyisocyanate for the production of light-stable polyurethanes, namely the readily obtainable hexamethylene diisocyanate, leads to high-quality elastomers, slightly clouded polyurethane urea solutions are formed during chain-extension with diamines in methyl methacrylate as solvent when using this diisocyanate. Although moldings produced from these solutions show excellent mechanical properties, they are unsuitable for the production of glass-clear sheets.